Respiratory disease is the major cause of human death, SO2 is a universal air pollutant, the nasal mucosa is the first site of impact of airborne viruses and toxins, and epithelial sloughing is a primary response to both. The most important single mechanism for maintaining nasal mucociliary clearance during exposure to daily fluctuating doses of toxins and viruses is mucociliary homeostasis. New techniques will be used to monitor nasal homeostatic mechanisms sequentially in individual living chickens during exposure to combined effects of acute and chronic viral infections and graded levels of SO2. Immunopathology and nasal histopathology will be followed during all phases of the interactions between viruses and pollutant gas. An added study will evaluate effects of continuous low level exposure to SO2 during the first three weeks of life on cell dynamics, on nasal and sinus transport rates, and on development of organized lymphoid cell systems in nasal and paranasal tissues and in the thymus and bursa.